This invention relates to a method for compressing and restoring data in a magnetic tape memory device, etc. and an apparatus for realizing same and in particular to a method for compressing and restoring data, which is capable of increasing the data compression ratio, and an apparatus for realizing same.
As a method for compressing and restoring data in a magnetic tape memory device according to prior art techniques there is known a method based on the so-called run-length method, by which a portion consisting of a repetition of identical patterns in byte unit is searched from data to be recorded and these successive identical patterns are compressed and recorded in a magnetic tape, as disclosed in U.S. Pat. No. 4,586,027.
According to the prior art technique described above, whether a portion in question is a repetition of identical patterns or not (hereinafter simply referred to as "successiveness") is judged in one-byte unit. Therefore, even though a series of data to be written is not successive when the successiveness is considered by one-byte unit, it is likely that it contains many portions which can be compressed by the above method if the successiveness is considered with respect to high-order 4 bit (half byte) groups or low-order 4 bit groups of the data series. For this reason, it can be said that the compression efficiency according to the prior art technique is not always high.